This application claims priority under 35 U.S.C. xc2xa7119 of Korea 56933/2001, filed Sep. 14, 2001, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The invention relates to an electron gun for a color cathode ray tube (CRT), and in particular to an electron gun for a color CRT having an improved electrode structure and shape in order to reduce a spot diameter influencing the electron beam focus.
2. Description of the Related Art
FIG. 1 is a schematic structure view illustrating a related CRT.
Referring to FIG. 1, a multiple electrodes are formed on an in-line electron gun for the CRT. The multiple electrodes are positioned at predetermined intervals in a vertical direction to a path of an electron beam 13, so that the electron beam 13 generated in cathodes 3 can reach a screen 17 at a predetermined strength.
In detail, the multiple electrodes include a first electrode 4 which is a common lattice of three cathodes 3, a second electrode 5 arranged at a predetermined interval from the first electrode 4, a third electrode 6, a fourth electrode 7, a fifth electrode 8 and a sixth electrode 9.
In addition, a shield cup 10 having a BSC (Bulb Space Contactor) 11 for electrically connecting the electron gun to the tube and fixing the electron gun to a neck unit of the tube is formed on the sixth electrode 9.
The operation of the electron gun will now be explained. In the electron gun, electrons are emitted from stem pins 1 by a heater 2 of the cathode 3. The electron beam 13 is controlled by the first electrode 4 which is a control electrode, accelerated by the second electrode 5 which is an accelerating electrode, and partially focused and accelerated by a full space focus lens formed among the second electrode 5, the third electrode 6, the fourth electrode 7 and the fifth electrode 8.
The electron beam is mostly focused and accelerated by the sixth electrode 9 which is synchronized with a deviation signal to form a quadrupole lens for compensating for astigmatism generated by a deflection yoke, which receives a variable voltage and which is a main lens formation electrode called a focus electrode, and the seventh electrode 10 which is an anode electrode. The focused electron beam 14 passes through a shadow mask 15 formed inside a fluorescent surface 16, and collides with the fluorescent surface 16, to emit light.
The deflection yoke 12 for deviating the electron beam 13 emitted from the electron gun to the whole surface of the screen 17 is positioned outside the electron gun, thereby embodying the screen.
A velocity modulation (VM) coil 18 synchronized with an image signal of a circuit is formed on the neck unit of the CRT having the related electron gun. In one major aspect of the invention, the VM coil 18 is used to reduce the spot diameter.
In design properties of the electron gun, a lens magnification, a space charge repulsive force and a spherical aberration of a main lens influence the spot diameter of the screen.
Such properties will now be explained in more detail.
The variation of the spot diameter Dx due to the lens magnification is rarely used as a design factor of the electron gun and less effective, since basic voltage conditions, a focal distance and a length of the electron gun are previously determined.
The space charge repulsive force is a spot diameter magnification phenomenon resulting from repulsion and collision of electrons of the electron beam. In order to restrict magnification of the spot diameter Dst due to the space charge repulsive force, a progressing angle of the electron beam (hereinafter, referred to as xe2x80x98divergence anglexe2x80x99; xcex1) is preferably set up to be increased.
In the spherical aberration property of the main lens, the spot diameter D1c is magnified due to difference in the focal distance between electrons passing through a radical axis of the lens and electrons passing through a protaxis thereof. Oppositely to the space charge repulsive force, when the divergence angle of the electron beam incident on the main lens is decreased, the spot diameter is embodied small on the screen.
As explained above, the spot diameter Dt on the screen is represented by the following formula as adding up of three factors:       D    t    =                              (                                    D              x                        +                          D              st                                )                2            +              D        ic        2            
Especially, a method for magnifying a diameter of a main lens has been suggested to reduce the space charge repulsive force and the spherical aberration.
Since the diameter of the main lens is magnified, if an electron beam having a large divergence angle is incident, magnification of the spots due to the spherical aberration is restricted, and the space charge repulsive force after passing through the main lens unit is decreased, thereby embodying small spots on the screen.
FIG. 2 is an experimental result showing variations of the spot diameter by the main lens diameter.
As shown in the graph of FIG. 2, when the diameter of the main lens is increased, the magnification of the spot diameter due to the spherical aberration of the main lens is restricted, thereby reducing the spot diameter on the screen.
Generally, the fifth electrode receiving a high voltage and the sixth electrode synchronized with the deviation signal for receiving a variable voltage are called focus electrodes. A length of the focus electrode is an important factor for determining a voltage ratio (%: focus/high voltage) of the electron gun. Here, the sixth electrode is used to compensate for astigmatism of the deflection yoke. When it is not necessary to improve resolution and clearness of a peripheral screen unit, the sixth electrode may not be employed.
A method for mechanically magnifying a hole diameter of a main lens formation electrode and a method for increasing a depth of an electrostatic field control electrode for correcting a lens have been taught as methods for magnifying a main lens unit.
It is almost impossible to mechanically increase the hole diameter of the electrode due to xcfx8629.1 mm of a neck diameter, and thus difficult to improve quality of focus.
Accordingly, a circuit of a chassis driving the CRT is appropriately controlled to reduce the spot diameter on the screen and improve resolution, so that a differential signal of an image signal for scanning the electron beam to the screen is synchronized with the coil of the neck unit of the CRT having the electron gun, and the electron beam is evenly modulated in a deviation speed by a deviation magnetic field of the deflection yoke. As a result, resolution and-clearness of the screen are improved.
FIG. 3 is a schematic view illustrating an operation principle of the VM coil.
In order to optimize the method for improving resolution on the circuit, a whole length of the electrode receiving a fixed focus voltage must be sufficiently short, and an interval must be sufficiently prepared so that a speed modulation magnetic field due to current synchronized with the image signal of the coil can be efficiently penetrated thereinto.
The structure of the related electron gun is not suitable to maximize the effect of the coil.
In general, a center of the magnetic field generated by the coil is positioned adjacently to the fifth electrode receiving a relatively high voltage. A length of the electrode is increased for the required voltage ratio in design.
As compared with alignment of small components, to increase the length of the electrode can cut down production expenses and simplify a production process.
However, the structure of the electron gun by the simplified long electrode deteriorates speed modulation effects due to the magnetic field of the coil, thus restricting improvement of the resolution.
It is, therefore, an object of the present invention to provide an electron gun which can maximize an operation of a VM coil synchronized with a differential signal of an image signal, by arranging at least two focus electrodes receiving a fixed focus voltage in a row among main electrodes including an anode electrode and cathode electrodes, and by maintaining an appropriate interval between the focus electrodes receiving the fixed focus voltage.
To achieve the above object, in a cathode ray tube having a cathode for emitting an electron beam to a fluorescent screen, an electron gun having an anode electrode of a screen side and focus electrodes of a cathode side, and a VM coil using a position of the focus electrode as a center of a magnetic field, being positioned on a neck unit of the cathode ray tube, and being synchronized with an image signal of a circuit, the electron gun including at least two focus electrodes of main electrodes receiving a fixed focus voltage, wherein, when a sum of lengths of the focus electrodes is xe2x80x98Lxe2x80x99 and a sum of intervals of the electrodes is xe2x80x98gxe2x80x99, xe2x80x98(gxc3x97100)/L=5xcx9c30(%)xe2x80x99 is satisfied.
In one aspect of the present invention, resolution of the screen is improved by minimizing a screen spot diameter by 15 to 30% in a horizontal direction.